The enzymes catalyzing the interconversion of proline and pyrroline-5-carboxylate provide a mechanism for the intercompartmental, intercellular, and interorgan transfer of redox potential. Mediated by the transfer of redox potential, PG regulates PPRP and nucleotide production thereby linking amino acid and nucleotide metabolism. This linkage provides an approach to enhance the antitumor activity of purine antimetabolites e.g., 6-mercaptopurine.